The number of times you can cast a Summon is the same as the number of Stars a Materia has - ie, at Level 1 (1 star) you can cast Phoenix once per battle.
I would presume it's the same for the Final Attack, with each star representing how many times in a single batle it will work.

I'm fairly sure it only goes upto a maximum of 5 only - unless you have Master Summon - but Final Attack will still only work up to 5 times at Master level.